1. Field of the Invention
This invention relates to aircraft and particularly to autogyro aircraft with a freewheeling rotor which serves as the lifting surface for the aircraft. More specifically, this invention relates to control arrangements including a system for maintaining the pitch or angle of attack of the blades of the freewheeling rotor.
2. Prior Art
An autogyro aircraft derives its lift from a freewheeling, unpowered rotor with horizontal thrust provided by a separate engine turning a standard fixed wing type aircraft propeller. The autogyro is neither an airplane nor a helicopter. Rather, it incorporates a freewheeling rotor in place of a fixed wing. An airflow passing upwardly through the rotor blades provides lift in autorotation. A second generation autogyro is provided for bringing the rotor blades up to flight speed with the aircraft on the ground.
U.S. Pat. No. 1,590,497 to a Juan de la Cierva of Madrid, Spain illustrates a first embodiment of an autogyro aircraft. De la Cierva was a prolific inventor, patenting a number of improvements in autogyro systems both in the United States and abroad and was issued at least nine United States patents in the area of autogyro aircraft. None of which patents, however, involved apparatus for providing selective control of rotor blade angle of attack as does the present invention.
The de la Cierva autogyro was followed by improved versions that were embodied in a number of patents as were issued to H. F. Pitcairn from 1927 to 1956 that taught improvements upon the de la Cierva design and ultimately evolved into a helicopter-type aircraft. As an evolution from the de la Cierva aircraft, H. F. Pitcairn developed and marketed an autogyro known as a PCA2. PCA stood for Pitcairn Cierva Aircraft.
The helicopter aircraft of Pitcairn had a powered rotor system with the capability for selectively changing the rotor blade angle of attack to provide a steering capability as well as a forward thrust component for forward flight. The arrangement for selectively changing rotor blade angle of attack for a turned rotor is commonly known as a swashplate and provides for altering the blade of attack through a full three hundred sixty degrees (360.degree.). Of course, a helicopter rotor structure must also have a capability for selectively tilting the rotor mast. This rotor mast tilting is from the vertical to an angle where, in conjunction with the action of the swashplate, there is provided a forward thrust component to move the vehicle horizontally as the rotor blades provide lift. A forward thrust component is not required for an autogyro rotor system, as the autogyro aircraft incorporates a separate propeller for this purpose.
Both de la Cierva and Pitcairn ultimately recognized the desirability of providing for changing, between limits, the rotor blade pitch or angle of attack for pre-takeoff spin up of the autogyro rotor. After spin up, the rotor blade angle of attack could be changed from a neutral non-lifting angle to a lifting attitude facilitating the aircraft lifting off after only a short takeoff roll distance.
In U.S. Pat. No. 1,947,901, de la Cierva to set the rotor blades in a positive lift angle of attack prior to turning the rotor. Similarly, Pitcairn, in U.S. Pat. No. 1,977,834, also shows a sleeve arrangement for pre-setting a rotor blade angle of attack. Shifting the rotor blade pitch angle during rotation of the blades was recognized by Pitcairn in U.S. Pat. No. 1,884,847. This capability, however, was between limits and, unlike the present invention, did not provide for incrementally changing rotor blade angle of attack prior to and during flight.
As noted earlier, the autogyro lead to and was replaced by the helicopter as a preferred aircraft. The helicopter replaced the free-spinning rotor of the autogyro with a driven rotor system that included structure for both canting the turning rotor mast as well as for altering the angle of attack of the rotor blades. A tail fan system was also incorporated to control yaw. These developments are shown in U.S. Pat. Nos. 2,247,034; 2,311,247; 2,350,126; 3,352,342; 2,352,404; 2,473,299; and 2,499,161.
With the development of the helicopter, a vertical takeoff and landing capability was achieved along with the ability to hover. Because the autogyro aircraft could not hover, the autogyro lost favor, and further development efforts ended. However, the autogyro has benefits which may be more appropriate for selected uses so that a practical autogyro, as contemplated by the present invention, is now viable.
Following the efforts of de la Cierva and Pitcairn to the present, autogyro aircraft improvements have essentially been refinements to early structure. For example, a patent by Salisbury, et al., U.S. Pat. No. 1,838,327, shows an early system for altering rotor blade angle of attack. U.S. Pat. No. 2,154,601 (Bennett) shows a system for changing rotor blade angle from a no-lift pitch to a lifting pitch as the rotor is rotated to takeoff speed. Like the earlier cited Pitcairn systems, this alteration of rotor blade pitch angle is accomplished between preselected limits rather than being incrementally controllable as with the present invention.
U.S. Pat. No. 4,092,084 (Barltrop) is also directed to a system for changing rotor blade pitch angle. Rotor blade pitch angle changes occur in relation to a rotor rotation rate. The blade angle changes between set limits. A rotor blade pitch angle control is also shown in U.S. Pat. No. 3,149,802, (Wigal). Like the Barltrop patent, Wigal shifts the rotor blade pitch angle between limits responsive to an increase in rate of rotation of the rotor. U.S. Pat. No. 3,465,705 (Bensen) is directed to a system for diverting engine power to rotate a rotor from rest to a takeoff rotation rate. The rotor blades cant between limits responsive to rotation rate of the rotor. U.S. Pat. No. 2,183,119 (Larsen) also shows a system for rotating a rotor from a rest position to a takeoff rotation rate.
None of the autogyro aircraft described in the above-referenced patents or autogyros known to applicants have a unidirectional control mechanism for variable control of an autogyro rotor in flight to control the pitch angle of the leading edges of oppositely disposed rotor blades and, in turn, the lift of an autogyro. U.S. Pat. No. 4,195,800 (Wallace) shows a helicopter collective system for changing the angle of attack of rotor blades of a rotor system during a period of rotation of the rotor and mast through a full circle of three hundred sixty degrees (360.degree.).
Summarizing, some autogyro aircraft heretofore known have an automated arrangement for pivoting the rotor blades from a no-lift to lift attitude on reaching a certain rotation rate. The rotor blades may be rotated either through linkage to an aircraft engine or by taxiing the aircraft under the urging of a conventional engine turning a propeller to spin up the rotor blade to a desired rotation rate to in turn provide takeoff lift. In such systems, centrifugal forces increase with the rotation rate and typically act upon a structure to cause the blade angle of attack to change between a first no-lift position and second lift or takeoff position. No system provides for selective control of rotor blade pitch angle or angle of attack independent of the rotor blade rotation rate.